The present invention relates to fuel injection control systems for engines such as engines of automobiles, and more specifically to fuel injection control systems for injecting fuel twice or more times in each engine operating cycle.
Japanese patent provisional publications Nos. 60-195347, 60-233353, 59-211731 and 59-29733 disclose fuel injection control systems designed to meet recent demands for better fuel economy, speedier response, higher output and further improvement in emission control in all operating conditions.
These conventional injection systems are arranged to determine the injection timing and the fuel injection quantity in accordance with engine rpm, engine load and other engine operating parameters, and advance the injection timing so that the fuel can be correctly introduced into the engine within the suction stroke even when the fuel is enriched.
In these systems, however, it is difficult or practically impossible to choose the injection timing so as to satisfy both of conflicting requirements in steady state and transient state because these system inject the fuel only once in each cycle. Therefore, these systems are unable to provide satisfactory engine stability and response in some cases. During steady state operation, it is required to sufficiently advance the fuel injection timing relative to the intake stroke in order to promote evaporation of the fuel and achieve satisfactory combustion. During transient state, on the other hand, it is desired to vary the fuel quantity so as to follow a change of the engine load immediately by retarding the fuel injection timing as late as possible insofar as it is not too late for the intake stroke. These conflicting requirements in the steady and transient states cannot be satisfied by these conventional systems.
Japanese utility model provisional publication No. 61-76143 discloses another example in which the fuel is injected twice in each combustion cycle of the engine. The injection control system of this example is arranged to set first and second injection timings and determines a quantity of the fuel supplied at the first injection timing in each cycle and a quantity of the fuel at the second timing in accordance with engine load and intake air temperature. In this system, however, changes in the required fuel quantity due to changes in the fuel injection timing and the engine load are not taken into consideration. Therefore, this system cannot follow up a change of the engine load between the instant of the first fuel injection and the intake stroke, so that a deficiency of the fuel supply is left when the engine load is increased.
When, for example, an intake air flow rate Qa is increasing as shown in FIG. 8, a fuel quantity Te corresponding to the intake air amount per one revolution of the engine is increased from a value Te.sub.1 obtained at the time of the first injection to a value Te.sub.2 obtained at the time of the second injection. In this case, the total fuel quantity per two-revolution cycle amounts to 2.times.Te.sub.2 whereas the actual supply per cycle amounts only to Te.sub.1 +Te.sub.2 which is smaller than 2.times.Te.sub.2 when Te.sub.1 and Te.sub.2 are directly used. Therefore, the fuel supply is deficient by a difference .DELTA.Te=Te.sub.2 -Te.sub.1.